1. Field of the Invention
The present invention relates generally to glass-ceramics, in particular to substantially transparent glass-ceramics containing a microstructure comprising nanocrystalline aluminogallate spinel crystals (Li(Ga,Al)5O8 and xe2x80x9cxcex3-(Ga,Al)2O3) and solid solutions between these as the major crystalline phase.
2. Technical Background
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. In general, the method for producing such glass-ceramics customarily involves three fundamental steps: first, melting a glass-forming batch containing the selected metallic oxides; second, cooling the melt to a temperature at least below its transformation range, while simultaneously forming a glass body of a desired geometry; and third, heating the glass body to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ. To develop nuclei in the glass, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time; although there are certain compositions that are known to be self-nucleating and thus do not require the development of nuclei. Thereafter, the temperature will be raised to temperatures where crystals can grow from the nuclei. The resulting crystals are typically uniformly distributed and fine-grained. Internal nucleation permits glass-ceramics to have favorable qualities such as a very narrow distribution of particle size and a highly uniform dispersion of crystals throughout the glass host.
Transparent glass-ceramics are known in the art, with the classic study relating to transparency being authored by G. H. Beall and D. A. Duke in xe2x80x9cTransparent Glass Ceramics,xe2x80x9d Journal of Material Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. In other words, transparency typically results from crystals less than 50 nmxe2x80x94preferably as low as 10 nmxe2x80x94in size, if there is a major refractive index difference between crystal and glass. Transparency in glass-ceramics, alternatively, can also be produced with crystals larger than 50 nm if the crystal birefringence and the index of refraction mismatch between the crystal phase and the glassy phase are both low. Transparent glass-ceramics, doped with transition elements can combine the optical efficiency of crystals with the flexibility of the forming of glass. For example, both bulk (planar substrates) and fiber forms can be fabricated from these glass-ceramics.
Recently, researchers have concentrated much effort to develop transparent glass-ceramics as hosts for transition metal ions. Transition metals have been used as optically active dopants in crystalline hosts because they fluoresce in the near infrared (700 nm to 2000 nm) region. Given the useful wavelength range and relatively wide bandwidth of many transition-metal dopants, much interest has arisen for their use in optical telecommunication applications, with the region from 1000 nm to 1500 nm being of particular interest. The current optical telecommunication medium is glass-based optical fiber. Inclusion of transition metal dopants into glasses, however, has unfortunately not produced fluorescence performances as good as in crystalline materials. The performance of transition metal ions tends to degrade in amorphous hosts, where the crystal field strength is much smaller than in even crystalline hosts.
Suitable glass-ceramic hosts, therefore, must be tailored such that transition elements will preferentially partition into the crystal phase. Some of these glass-ceramics have come from compositions such as those discussed the following applications. Co-pending U.S. patent application, Pub. No. 2002/0028739, entitled FORSTERITE GLASS-CERAMICS OF HIGH CRYSTALLINITY AND CHROME CONTENT, by George H. Beall, et al., and co-pending U.S. Pat. No. 6,300,262, entitled TRANSPARENT FORSTERITE GLASS-CERAMICS, by George H. Beall both of which disclose a family of, and a method of making, glass compositions based in the K2 Oxe2x80x94xe2x80x94MgOxe2x80x94xe2x80x94Al2O3xe2x80x94xe2x80x94SiO2 system. U.S. Pat. No. 6,297,179, entitled TRANSITION-METAL GLASS-CERAMIC GAIN MEDIA, by George H. Beall et al., discloses transition-metal-doped glass-ceramic materials used as gain media or pump laser fiber in optical amplifiers and lasing mechanisms. WO 01/28944 entitled TRANSPARENT LITHIUM ORTHOSILICATE GLASS-CERAMICS, by George Beall, et al., discloses a family of glass compositions within the ternary Mg2SiO4xe2x80x94Zn2SiO4xe2x80x94Li4SiO4 system and exhibiting a predominate orthosilicate crystal phase. Lastly, U.S. Pat. No. 6,303,527 entitled Transparent Glass-ceramics Based on Alpha- and Beta-Willemite, by L. R. Pinckney discloses substantially and desirably totally transparent glass-ceramics, and which contain a willemite predominant crystal phase within the ternary Mg2SiO4xe2x80x94xe2x80x94Zn2SiO4xe2x80x94xe2x80x94Li4 SiO4system. Each of these patents and applications are co-assigned to the present assignee and the entire contents of both of these applications are incorporated herein by reference.
Transparent glass-ceramics which contain relatively small numbers of crystals can be of great use in cases where the parent glass provides an easy-to-melt or an-easy-to-form vehicle for a crystal. The single crystals may be difficult or expensive to synthesize, however they provide highly desirable features, such as optical activity. The crystals in the glass-ceramic are generally oriented randomly throughout the bulk of the glass contrary to a single crystal which has a specific orientation. Random orientation, and consequent isotropy, are advantageous for many applications. One example is that of optical amplifiers, where polarization-independent gain is imperative.
Transparent glass-ceramics doped with transition elements can combine the optical efficiency of crystals with the forming flexibility of glass. For example, both bulk (planar) and fiber forms can be fabricated from these glass-ceramics.
Transparent, transition metal-doped spinel glass-ceramics, particularly doped aluminate and gallate spinels, are known in the art. The optical properties of both aluminate and gallate spinel crystals, when doped with various transition metal ions, have also been described in the literature. Various potential applications have been disclosed, including photoluminescent phosphors as well as tunable solid state lasers and saturable absorbers for visible and near-infrared wavelengths. These studies have been carried out on single crystals or polycrystalline powders, as well as Cr3+-doped ZnAl2O4 and LiGa5O8 glass-ceramics and a Co2+-doped ZnAl2O4 spinel glass-ceramic; in these studies the optical properties of Co2+-doped aluminate (ZnAl2O4) and gallate (LiGa5O8) spinel glass-ceramics were compared. The latter material comprised a doped ternary (Li2Oxe2x80x94Ga2O3xe2x80x94SiO2) bulk glass composition and a microstructure of LiGa5O8 crystals dispersed in a silicate glass. Although the aluminate and gallate spinel glass-ceramics described above are suitable as potentially valuable hosts for small, optically active transition elements, and thus, as doped, exhibit luminescence and/or fluorescence, these gallate ternary glasses are somewhat difficult to melt and form in bulk quantities into usable forms suitable for application in the optical field industry.
Accordingly, the primary object of the present invention is to provide glass-ceramic material which is substantially and desirably totally transparent which is capable of being more easily melted and formed into optical articles than the aforementioned prior art glass-ceramics.
Another object of the present invention is to provide substantially and desirably totally transparent nanocrystalline aluminogallate spinel glass-ceramics which are capable of being doped with ingredients which confer useful optical properties including, but not limited to, luminescence and/or fluorescence, and broadband absorption.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, one aspect of the invention disclosed is a substantially transparent glass-ceramic ceramic exhibiting an aluminogallate spinel crystal phase and having a glass-ceramic composition that lies within the SiO2xe2x80x94Ga2O3xe2x80x94Al2O3xe2x80x94K2Oxe2x80x94Na2Oxe2x80x94 system and particularly consisting essentially, in weight percent on an oxide basis, of 25-55% SiO2, 9-50% Ga2O3, 7-33% Al2O3, 0-20% K2O, 0-15% Na2O, 0-6 Li2O and 5-30% K2O+Na2O, the glass ceramic microstructure containing a crystal phase comprising at least 5%, by weight, of aluminogallate spinel crystals.
One additional aspect of the invention relates to an optical element selected from the group consisting of an optical fiber, a gain or laser medium, and an amplifier component, a saturable absorber, the element comprising: a transparent glass-ceramic containing a crystallinity of at least about 5% by weight of aluminogallate spinel crystals, the glass-ceramic having a composition consisting essentially, in weight percent on an oxide basis, of 25-55% SiO2, 9-50% Ga2O3, 2-35% Al2O3, 0-20% K2O, 0-15% Na2O, 0-6 Li2O and 5-30% K2O+Na2O.
In another aspect, the present invention relates to a method of making a substantially transparent glass-ceramic containing an aluminogallate spinel crystal phase and having a glass composition that lies within the SiO2xe2x80x94Ga2O3xe2x80x94Al2O3xe2x80x94K2Oxe2x80x94Na2Oxe2x80x94 system, the method comprising the steps of:
a.) melting a batch for a glass having a composition consisting essentially, in weight percent on an oxide basis, of 25-55% SiO2, 9-50% Ga2O3, 2-35% Al2O3, 0-20% K2O, 0-15% Na2O, 0-6 Li2O and 5-30% K2O+Na2O;
b.) cooling the glass to a temperature at least below the transformation range of the glass;
c.) exposing the glass to a temperature between about 550-950xc2x0 C. for a period of time sufficient to cause the generation of a glass-ceramic which is substantially transparent and which contains a predominant, aluminogallate spinel crystal phase whose glass composition lies within the SiO2xe2x80x94Ga2O3xe2x80x94Al2O3xe2x80x94K2Oxe2x80x94Na2O system; and,
d.) cooling the glass-ceramic to room temperature.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.